If the site can detect that it can't use WebUSB, it can give you instructions on how to download and flash the mobile OS, not tell you to fuck off.

Compare: https://wiki.lineageos.org/devices/tokay/

That's not an installer, that's a device page.

It's the specific functionality needed here that Firefox lacks that makes the /e/ page show the warning, unlike the lineage page that does not have the problem in the first place.

The fun part is that I got to this installer page by clicking on "Check device compatibility" on the https://e.foundation/e-os/ page..

So I was actually expecting a device page, not a WebUSB program..

Okay, that makes your complaint very understandable.

Absolutely. This is handled very badly, and I was also surprised about the bad UX on that screen.